Many image-forming apparatuses such as printers have fixing units.
Such a fixing unit fixes an unfixed toner image that has been formed by an imaging process, e.g. electrophotographic, electrostatic, and magnetic recording, on recording materials, e.g. a recording sheet, sensitized paper, and electrographic paper. Fixing units known as such employ a heating roller method and a film heating method. In recent years, an image-forming apparatus having a fixing unit using an electromagnetic induction heating method is known.
A fixing unit using the film heating method is disclosed in Japanese Patent Application Non-Examined Publication No. S63-313182 or No. H01-263679, for example.
As for a fixing unit using the electromagnetic induction heating method, a technique of causing a fixing roller to produce heat resulting from magnetic induction is disclosed in Japanese Patent Application Non-Examined Publication No. H11-297462. Herein, an alternating field causes the conductive layer of the fixing roller to produce eddy current and thus Joule heat, and this Joule heat causes the fixing roller to heat.
A fixing unit using the electromagnetic induction heating method is described below.
FIG. 22 is a schematic view of a fixing unit using a conventional electromagnetic induction heating method.
In the fixing unit shown in FIG. 22, exciting coil 42 is disposed along outer circumferential surface of fixing roller 41. Magnetic substance 43 is disposed outside of and over this exiting coil 42. Pressurizing roller 44 is disposed so as to press fixing roller 41 in contact therewith. Temperature sensor 45 detects temperatures on the surface of fixing roller 41.
Alternating current at frequencies of 10 to 100 MHz is applied to exciting coil 42. The magnetic field induced by this alternating current feeds eddy current through the conductive layer of fixing roller 41, thereby causing Joule heat.
Temperature sensor 45 is disposed in contact with the front face of fixing roller 41. Responsive to signals detected by temperature sensor 45, power supply to exciting coil 42 is increased or decreased. Thus the temperatures on the front face of fixing roller 41 are automatically controlled so that a predetermined fixed value is maintained.
Recording material 46 carrying unfixed toner image 47 thereon is conveyed and placed by a carrier guide (not shown) in a position in which the recording material is guided to a nip portion “NI” between fixing roller 41 and pressurizing roller 44.
In this manner, fixing roller 41 is rotated by a driving unit (not shown). At the same time, alternating current is applied to exciting coil 42 to heat up fixing roller 41. Therefore, the fixing nip portion “NI” is heated to a predetermined temperature. In this state, recording material 46 carrying unfixed toner image 47 thereon is guided by the carrier guide (not shown) and introduced into the fixing nip portion “NI”. The recording material is further conveyed as fixing roller 41 rotates, and toner image 47 is melted and fixed onto recording material 46 by the heat of fixing roller 41 and the pressure of the nip portion.
As mentioned above, a fixing unit using the electromagnetic induction heating method heats fixing roller 41 with high heat transfer by utilizing eddy current produced by electromagnetic induction. Therefore, this method has such advantages as reducing warm-up time, allowing the unit to start earlier than a fixing unit using the film heating method, and contributing to energy saving.
In Japanese Patent Application Non-Examined Publication No. H08-286539, the following structure is disclosed.
A rotating heat-producing section has a conductive layer comprising a film containing a ferromagnetic metal, e.g. nickel, iron, ferromagnetic stainless steel, and nickel-cobalt alloy. Provided inside of the rotating heat-producing section is an electromagnetic induction heating section that has exciting coils wound along a core material in the direction of the rotating shaft of the rotating heat-producing section.
A fixing unit using the electromagnetic induction heating method disclosed in Japanese Patent Application Non-Examined Publication No. H11-297462 is structured so that the unit has an electromagnetic induction heating section outside of a fixing roller and substantially half of a circumferential area of the fixing roller is locally heated. In order to prevent abnormal temperature rise in a heat-producing section resulting from uncontrollable temperature, a heat controller comprising a heat-sensitive operation section, such as a thermostat, is provided in a position opposite to the electromagnetic induction heating section, i.e. inside of a heat-up roller.
In this structure, the surface of the heat-sensitive operation section may be worn by sliding thereof resulting from the rotation of the fixing roller. Therefore, it is difficult for the heat-sensitive operation section to be pressed onto the inside of the heat-up roller in contact therewith in a stable manner.